Solid polymer fuel cells that have been in general use until the present are composed of a plurality of cells stacked together. Each cell includes: a membrane electrode assembly (MEA) composed of an electrolyte membrane [which is made of an ion exchange membrane], a fuel electrode (an anode) [which is made of a catalyst layer and a diffusion layer placed on one side of the electrolyte membrane], and an oxidizer electrode (a cathode) [which is made of a catalyst layer and a diffusion layer placed on the other side of the electrolyte membrane], and a separator that forms a fluid passage for supplying a fuel gas (hydrogen) to the fuel electrode, and an oxidizing gas (oxygen [or usually the air]) to the oxidizer electrode.
In a fuel cell system including the fuel cell described above, hydrogen as the fuel gas and water generated by a cell reaction flow through a hydrogen circulation system. Since not all the supplied hydrogen is used for the cell reaction, this fuel cell adopts a circulation system that effectively utilizes that un-reacted hydrogen by supplying it back to the fuel cell. The water generated by the cell reaction is discharged outside. In this type of hydrogen circulation system, a pump is usually mounted in a passage as the circulating power source.
Minute amounts of components dissolving from, for example, piping parts of the fuel cell or the system exist in the water flowing through the hydrogen circulation system. Also, impurities in the air drawn in from the outside may enter the passage, pass through the electrolyte membrane, and be mixed in the hydrogen circulation system. In particular, if metal ions exist in the components dissolving from, for example, piping parts of the fuel cell or the system, there is the possibility that the functions of the fuel cell itself may be degraded and the service life of the fuel cell may be shortened. In addition, the water generated in the fuel cell may become acidic.
A method of using an ion exchange resin has been generally employed as a method for purifying water that flows through a hydrogen circulation system like the one described above. If this fuel cell system is to be mounted on an automobile or similar, an extra loading space is needed. It is also necessary to regularly change the ion exchange resin. Accordingly, downsizing the fuel cell system and extending the cycle for changing the ion exchange resin is necessary.
As an example of a system for purifying water generated inside a fuel cell, using an ion exchange resin, JP-A-8-298130 describes a fuel cell system in which a filter for removing the impurities contained in a fuel gas is placed downstream from a junction with a cathode recycle blower discharge pipe for allowing a gas from a cathode outlet to circulate, thereby removing impurities such as iron molds and salts contained in the cathode gas.
JP-A-2001-313057 suggests a method for manufacturing an ion exchange filter that removes impurities contained in a gas supplied to a fuel electrode and an oxidizer electrode, by treating the surface of a base filter made of polyolefin or polyfluoroolefin to make it hydrophilic, and applying an ion exchange polymer solution to the base filter, and then drying the ion exchange polymer solution applied on the base filter.
JP-A-2002-313404 suggests a solid polymer fuel cell system equipped with an ion removal unit for removing ions contained in water that is generated by a fuel cell and accompanied by an exhaust gas, wherein the ion removal unit is placed, on the side closer to the solid polymer fuel cell, at either a fuel gas exhaust pipe or an oxidizer gas exhaust pipe, or both of them, from which the water generated by the fuel cell is discharged.
Moreover, JP-A-2001-35519 suggests a fuel cell coolant circulation system in which a cartridge-type ion exchanger is provided on a fuel cell coolant circulation line for a fuel cell mounted on a mobile body; two filters are placed opposite to each other in the ion exchanger; and one of the filters is equipped with a porous plate and a spring for applying a force to the porous plate in the direction toward the other filter (in other words, for pressing the porous plate in the axial direction of the cartridge-type ion exchanger). The spring is provided in the flow path of the coolant. Even if the volume of the ion exchange resin changes (particularly if the ion exchange resin contracts) during use, this coolant circulation apparatus can appropriately maintain the ion exchange resin firmly between the filters because the axial-direction pressing means composed of the spring and the porous plate applies pressure to compress the ion exchange resin in the axial direction.
However, the filter provided in the fuel cell system described in JP-A-8-298130 is used to remove impurities such as iron molds and salts contained in the cathode gas, and is not intended to securely remove impurities contained in moisture particles mixed in the cathode gas.
The use of the ion exchange filter obtained by the manufacturing method described in JP-A-2001-313057 in an exhaust gas passage for allowing an exhaust gas from the fuel cell to flow through is not mentioned. No attention is paid to the removal of impurities contained in the moisture particles mixed in the exhaust gas.
Moreover, the ion removal unit provided in the fuel cell system described in JP-A-2002-313404 is used to remove ions contained in the generated water flowing through pipes, and is not intended to remove impurities contained in the moisture particles mixed in the exhaust gas discharged from the fuel cell.
Furthermore, in the fuel cell coolant circulation apparatus described in JP-A-2001-35519, the cartridge-type ion exchanger is provided on the coolant circulation line for the fuel cell. Accordingly, no attention is paid to placement of the cartridge-type ion exchanger in an exhaust gas passage for allowing an exhaust gas from the fuel cell to flow through. In particular, no attention is paid to placement of the cartridge-type ion exchanger in the hydrogen circulation system. Therefore, the fuel cell coolant circulation apparatus is not designed to have the cartridge-type ion exchanger placed at a position in a gas-liquid separator where the ion exchanger would not disturb efficient separation between liquid and gas.